Magnetic resonance imaging and spectroscopy at the nanoscale via probe paramagnetic centers

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In this project funded by the Chemical Measurement and Imaging program, Carlos A. Meriles of the City University of New York (City College) is developing magnetic resonance imaging, or MRI, techniques for a variety of materials, including living tissue, with much higher resolution than is currently available. MRI is a well-known technique in diagnostic medicine but it is limited in the size of structures within the body that can be visualized. Recent advances have made it possible to visualize small organelles within individual cells, but the ability to probe within these organelles still eludes researchers. This project seeks to refine MRI in order to acquire sharp images of ever-smaller objects without going to invasive means. The research is focused on the development of a new type of probe for the MRI device. The probe, consisting of a tiny tip that can be hovered over the sample being studied, is coated with a special kind of diamond in which two adjacent carbon atoms have been removed. A nitrogen atom replaces one of these carbons, but the other is left as an empty hole. The resulting defect, known as a nitrogen vacancy, or NV, defect, responds to variations in the sample being studied by changing the way it spins. The change in spin direction can be detected by light that is emitted from this tip after it has been illuminated by a laser. The emitted light is stronger if the NV center spins around an axis pointing up, but it is weaker if the spin is around an axis pointing down toward the sample. The difference in emitted light in different parts of the sample can be fed into a computer and converted into an image that has a much higher resolution than current MRI images. It is possible that one day this technique may even allow scientists to visualize single atoms within large biological molecules such as proteins. This work is, thus, having a broad impact through the development of tools that will find wide applicability in biological science and medicine. It is having a further broad impact through the development of opportunities for underprivileged students to participate in this research through summer activities at the City College of New York as well as in the host laboratories of partner institutions. The project addresses a current limitation of MRI by exploring a new modality of spin sensing at the nanoscale via the use of a nitrogen vacancy (NV) centers in diamond. Rather than detecting single spins, the strategy focuses on the case where the NV center interacts with small ensembles of spins localized over effective volumes of about a hundred cubic nanometers. Building on the group's recent observation of proton spin noise at the nanoscale, the current goal is to advance NV-based spin sensing via new protocols designed to enhance the information content of the observed signals and broaden the technique's applicability to a more general class of samples. The work has two main thrusts: (1) The first thrust area uses near-surface NVs to probe model sample systems on the diamond surface whose molecular dynamics is changed by inducing a controlled phase transition or by restricting diffusion. Various magnetic resonance schemes are being implemented so as to expose the composition, mobility and, if possible, the structure of the sample molecules via spectroscopic signatures. (2) The second main thrust involves a shift in emphasis from nanoscale spectroscopy to nanoscale imaging via a geometry based on an NV-hosting scanning tip. A unique set of high-purity diamond nanopillars produced via top-down nanofabrication is being combined with an AFM-confocal system to demonstrate T1-weighted spin imaging with nanoscale spatial resolution. Since the presence of additional paramagnetic defects in the diamond host is not necessarily detrimental to sensing, the work has another goal, closely related to the first, to explore alternate protocols conceived to initialize and control the spin bath in a broad set of engineered nanocrystals.